This invention pertains generally to antennas for radio frequency energy and particularly to antennas wherein a planar array of slotted waveguides is used.
It has been common practice in the art of designing radar antennas for seekers in guided missiles to use a so-called resonant slot array. According to known practice, such an array is formed by mounting a plurality of similarly dimensioned slotted rectangular waveguides in proximity with one another to cover a predetermined aperture. An electrical short circuit is formed across one end of each waveguide to make a resonant structure wherein standing waves may exist to optimize the energization of the slots. A corporate feed of conventional design then is connected to the second ends of the waveguide to allow operation of the resonant slot array either as a transmitting antenna or a receiving antenna, such as a monopulse antenna. An example of the arrangement of the slots in a known slotted array is shown in U.S. Pat. 4,038,742.
Ordinarily, when a resonant slot array is to be used in a guided missile, it is necessary that: (1) a broadside pencil beam be formed so that antenna gain is maximized, with sidelobe levels as low as possible; and (2) the energy in the beam be linearly polarized, with cross-polarization effects minimized. In order to achieve the foregoing in the limited space available in the cylindrical body of a guided missile, the aperture of the usual slot array is circular in shape, the array itself is mounted so as to be steerable in pitch and yaw and the orientation of all of the slots with respect to the longitudinal axes of the waveguides is maintained constant. Further, if the slot array is to be operated as a monopulse antenna, the number of waveguides and disposition of slots is such that an equal number of slots is located in each quadrant of the aperture. In addition, the constraints on any slot array which must be met to avoid grating lobes or reduction in efficiency must be observed. That is to say, for a given frequency of operation, proper attention must be given to the dimensions of the waveguides, the spacing between slots and the position of the electrical short circuit in each one of the waveguides. Thus, in a typical application wherein the aperture of an antenna in a guided missile may have a diameter of 5", a slot array for X-band may have a maximum of 20 slots when known techniques are used to design such an array. Because the antenna gain of any slot array is directly related to the number of slots, the antenna gain of the array is limited.
Another problem is encountered with the conventional slot array wherein similarly dimensioned waveguides are used. Because the positions of the slots in each waveguide (along the length of such guide) are fixed, it is difficult to produce a symmetrical pencil beam. As a result, the quality of performance of the conventional slot array varies, depending upon the direction of a target from boresight.